IN Phyllida Lloyd’s impeccably measured biopic, former British prime minister Margaret Thatcher (Meryl Streep) is a woman who’s been surrounded by voices throughout her life. Some are supportive, many are vindictive; others are merely imagined, products of the stresses of office and, finally, of the dementia that has beset her in old age. Often they speak to her on screen. But frequently they’re like echoes in her consciousness, coming from another place. Lloyd’s skilful use of stereo sound effects constantly draws attention to these voices, as they cajole or confront her and she struggles to go her own way.

The movie’s protagonist is both “Margaret’’ and “Thatcher.’’ As it begins, she’s an old lady, tut-tutting about the price of milk (49 pence) and chatting with her ever-supportive and long-suffering husband, Denis (Jim Broadbent), over breakfast. Except he’s not there. She’s living comfortably but not luxuriously, surrounded by what’s left of her life and an armed guard, while under the care of nursing staff. Her daughter, Carol (Olivia Colman), visits occasionally; her son, Mark, briefly glimpsed in a treasured home movie shot while the family was on holiday in Cornwall in 1959, lives in South Africa. Her aloneness is palpable and heartbreaking.

Written by Abi Morgan (writer and creator of the gripping BBC TV series “The Hour”), Lloyd’s film moves back and forth between the present and the past, as if guided by this old lady’s fractured memories.

It’s revealed that the Iron Lady wasn’t always so, young Margaret’s ascension into the ranks of parliamentarians linked to a nascent feminism. Spurred on by the words of her small-businessman father (Iain Glen) — “Never go with the crowd, Margaret. Go your own way’’ — she does what she has to do to achieve her victory. She refuses to succumb to the local Conservative Party boys’ club’s condescension or to their whispering campaign against her. Only Denis (played as a young man by Harry Lloyd) stands by her.

Beautifully portrayed with a convincing blend of vulnerability and steely-eyed determination by Alexandra Roach, Margaret gradually morphs into Thatcher. Her transformation is born both of the times and of perceived necessity. And if she’s a “monster,’’ as an angry protester later describes her, the patriarchy she’s rebelled against is her Dr. Frankenstein.

Streep is truly extraordinary as the Thatcher that Margaret becomes. Her performance might initially seem to be merely a case of expert mimicry: Streep gets the voice, the intonation, the posture and the movement with breathtaking precision. But then she also takes us far beyond it, making her character a tragic figure, driven by her idealism (however misplaced) and destroyed by her blindness to her own failings and to the social breakdown that is a direct consequence of her decisions.

She is presented as a woman in denial. “Feelings do not interest me,’’ she haughtily declares. “Thoughts and ideas are what matter the most. What we think is what we become. My father always said that. And I think I am fine.’’ Beware of pity, she might well have said, for it can bring one undone. And, with her world collapsing around her — the Cabinet boys’ club sharpening their knives against her, and rightly so this time — she remains oblivious to why it’s happening.

It scarcely needs to be said that to try to understand the forces that shaped the controversial Thatcher is not to endorse any of her policies as Britain’s first and only female PM. But to be able to find compassion for this seemingly pitiless woman, as “The Iron Lady does,” is to rise above her limitations.